Aerials



Aug. 9, 1955 E. c. CORK 2,715,184

AERIALS Filed Sept. 24, 1 47 Mum/Ear;

EDWARD CECIL CORK United States Patent AERIALS Edward Cecil Cork, Eaiing, London, Engiand, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application September 24, 1947, Serial No. 775,783

Claims priority, application Great Britain October 1, 1946 Claims. (Cl. 25033) The present invention relates to resonant aerials for radiating or receiving electromagnetic waves over a wide range of frequencies.

For some purposes, particularly in television, an aerial is required which is capable of radiating or receiving electromagnetic waves whose frequency spectrum extends over a wide range. When resonant aerials are used for such purposes, it is necessary so to construct them that they have a comparatively poor selectivity. Thus, in aerials such as dipole aerials, it is common for this purpose to increase the diameter of the aerial conductor or conductors by employing tubular conductors, rather than single solid wires. By increasing the diameter of the conductors in this way, the inductance of the aerial is diminished and the capacity of the aerial is increased, so that the characteristic impedance and with it the selectivity of the aerial is reduced. If, however, an attempt is made, greatly to reduce the selectivity of the aerial in this way, by further increasing its diameter, it is necessary to shorten the length of the aerial in order to obtain the desired resonance. As a consequence of this shortening, the radiation resistance of the aerial is diminished. Such diminution results in a tendency to increase the selectivity of the aerial. In an example, it was required to provide a dipole suitable for radiating waves with substantially the same efficiency in the frequency spectrum extending from 50 to 70 megacycles per second. In attempting to obtain such a wide pass range, a diameter of aerial conductor of the order of 56 centimetres was employed, but it was found that the length of the dipole had to be so shortened that the radiation resistance of the dipole was reduced from the normal value of about 70 ohms to the very low value of 18 ohms and, moreover, a pass-range of only about 5 mega cycles or 8 per cent. of the centre frequency was achieved. Thus the reduction of radiation resistance rendered it impossible to achieve the desired wide passrange of frequencies. It is the object of the present invention to provide an improved aerial with a view to overcoming the above-mentioned difiiculties.

According to the present invention, there is provided an aerial coupled with energizing or receiving apparatus arranged to operate in a wide range of frequencies, said aerial comprising a thin conductor, a plurality of series impedances disposed at intervals along the length of said conductor, said intervals being smaller than one-quarter of the operating wavelength, and said conductor having such a length and the impedances having such magnitude that the aerial is caused to function with substantially uniform efficiency over said range of frequencies like a vibrator having a length equal to a number of quarter wavelengths, said number including one.

By operation in a wide range of frequencies with substantially uniform efficiency is meant operation over a band of frequencies not less in extent than 8 per cent of the mean frequency in the range and with variation of input impedance in said band not greater than would prevent said aerial being fed directly from a constant imice pedance source matched to the aerial at the centre of the band so that at no point in said band a reflection co-eificient in excess of 10 per cent is developed between said source and said aerial.

If desired, said aerial may comprise a pair of conductors for operating as a half wavelength resonator, said conductors being inclined to one another to impart a more sharply defined directive diagram.

An aerial in accordance with the invention is longer than a simple dipole and hence the aerial has a more extended current distribution than that of a simple dipole. Such an aerial tends to be less selective than a simple dipole since on the one hand the greater extension of current referred to above tends to yield a greater radiating efficiency so that the aerial is more damped. On the other hand, it also tends to be less selective because the aerial tends to be less reactive due to the neutralizing effect on the inductance of the aerial of the capacitative loading. The aerial also has a greater directivity than a simple dipole on account of the more extended current distribution.

If desired, the capacity loading may be so chosen that the current flow in the region of the extremities of the aerial is augmented.

In order that the invention may be clearly understood and readily carried into efiect it will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a capacity loaded dipole in accordance with one embodiment of the invention, and

Figure 2 shows another capacity loaded aerial in accordance with another embodiment of the invention in which the capacity loading is graded along the arms of the aerial to obtain an augmentation of current at the extremities of the aerial.

Referring to Figure l, the aerial shown comprises a pair of radiating or receiving conductors 10 and 11 which lie, for example, in a vertical vplane, and which may be each two free-space wavelengths in length and may be each inclined in said plane at 30 degrees with the vertical. Numerals 12 indicate series capacities inserted in the conductors 10 and 11. Said capacities may or may not be all equal in magnitude and uniformly distributed. It will be observed that the conductors 10 and 11 are nonterminated, that is they are not terminated by their characteristic impedances.

In one form of aerial constructed in accordance with the invention to operate with a centre frequency of 200 megacycles, each conductor 10 and 11 may extend for a length of two free-space wavelengths, and said capacities may be disposed at uniform intervals of A of a freespace wavelength and graded in magnitude in a uniform manner from 5 micro-micro-farads at the innermost ends of said conductors to 8 micro-micro-farads at the extremities thereof. The diameter of the conductors may be approximately 4 inch. Such conductors will have an impedance when unloaded of approximately 400 ohms and thus the impedances of the capacities aforesaid are small compared with the impedance of the unloaded conductors.

The conductors 11 and 12 can be connected at their adjacent ends 13, 14 to feeding or receiving apparatus operative in a wide range of frequencies.

With such an arrangement the aerial operates substantially as a half-wavelength resonator in said Wide range of frequencies substantially a quarter wavelength of current distribution existing along each conductor 10 and 11 of the aerial.

The directive diagram of the aerial is similar in character to the two lobes of a dipole except that the lobes are more sharply defined. If the conductors 10 and 11 are co-linear the diagram is symmetrical but each lobe may show a tendency to split into two lobes if the conductors and 11 are excessively long. By inclining the conductors 10 and 11 towards one another as shown in Figure 1, the tendency for the lobes to split is overcome but the directive diagram is rendered slightly asymmetrical.

Referring to Figure 2, there is shown a further embodiment of the invention in which the aerial is formed of a plurality of tubular conductors 15. These elements are insulated from one another and as indicated in Figure 2 are capacitatively coupled together by overlapping their ends. By correctly choosing the length of the individual tubular conductors or their degree of overlap or both a non-uniform capacitative series loading of the aerial can be achieved giving a half-wavelength current distribution along the aerial in which more current flows in the region of the extremities of the aerial than if the aerial were in the normal form of an unloaded straight rod, wire or tube, that is to say, in the form of a simple dipole.

If desired, an aerial according to the invention may consist of a straight wire loaded by series connected capacities distributed non-uniformly in magnitude or spacing or both so as to give augmentation at the extremities of the aerial. The aerial shown in Figure 1 of the drawings can, as stated, be connected to feeding or receiving apparatus at the adjacent ends 13 and 14 and likewise the aerial shown in Figure 2 can also be connected at the centrally disposed conductor 15 to feeding or receiving apparatus. Alternatively, the extremities of the aerials may be connected to said feeding or receiving apparatus.

If desired, in order to provide for de-icing the aerial, the capacities may be shunted by a line or coil present ing a high impedance for operating frequencies of the aerial but allowing the aerial to have a continuous path for D. C. or low frequency heating current.

Although the invention has been described above as applied to dipole aerials, it will be appreciated that the invention is also applicable to unipole aerials.

What I claim is:

1. An aerial coupled with apparatus for operation in a wide band of frequencies extending over at least 8 per cent of the mean frequency in the range of said band, said aerial comprising a thin conductor having one end coupled to said apparatus and its other end left free, a plurality of series capacities having small impedances at the operating wavelength compared with the impedance of said conductor without said capacities, said capacities having magnitudes increasing towards the free end of said conductor and being disposed along the length of said conductor at intervals smaller than one quarter of the operating wavelength, said intervals and the magnitudes of said capacities being proportioned to tune said aerial for operation in said band with a reflection coefficient of less than 10 per cent.

2. An aerial coupled with apparatus for operation in a wide band of frequencies extending over at least 8 per cent of the mean frequency in the range of said band, said aerial including a pair of arms inclined to one another and each comprising a thin conductor, adjacent ends of said conductors being coupled to said apparatus and their outer ends being left free, a plurality of series capacities having small impedances at the operating wavelength compared with the impedance of each con- 4 ductor without said capacities, said capacities having magnitudes increasing towards the free end of each conductor and being disposed along the length of each conductor at intervals smaller than one quarter of the operating wavelength, said intervals and the magnitudes of quency in the range of said band, said aerial comprising a thin conductor having one end free, a plurality of series capacities having magnitudes increasing towards the free end of said conductor disposed along the length of said conductor at intervals limited substantially to a maximum of one-twelfth of the operating wavelength, the magnitudes of said capacities and their spacing being proportioned to cause said aerial to operate with a reflection coefiicient of less than 10 per cent.

4. A tuned aerial for operation over a band of frequencies extending over at least 8 per cent of the-mean frequency in the range of said band, said aerial comprising a thin conductor having one end free, a plurality of series capacities having magnitudes increasing towards the free end of said conductor disposed along the length of said conductor at intervals limited substantially to a maximum of one-twelfth of the operating wavelength, the magnitudes of said capacities and their spacing being proportioned to cause said aerial to operate with a reflection coefiicient of less than 10 per cent, said conductor comprising a plurality of sections, and said sections having adjacent overlapping ends and means for insulating said overlapping ends to provide said capacities.

5. A tuned aerial for operation over a band of frequencies extending over at least 8 percent of the mean frequency in the range of said band, said aerial comprising a thin conductor having one end free, a plurality of series capacities disposed along the length of said conductor at intervals smaller than one quarter of the operating wavelength, said capacities having magnitudes increasing towards the free end of said conductor, the magnitudes of said capacities, and their spacing being proportioned to cause said aerial to operate with a reflection coefiicient of less than 10 per cent.

References Cited in the file of this patent UNITED STATES PATENTS 1,790,646 Alexanderson Feb. 3, 1931' 1,839,426 Von Arco Jan. 5, 1932 1,935,373 Peterson Nov. 14, 193.3 2,208,749 Cork et a1 July 23, 1940 2,258,406 Carter Oct. 7, 1941 2,272,608 Hoffman Feb. 10, 1942 2,276,910 Alford Mar. 17, 1942 2,352,977 Scheldorf July 4, 1944 2,409,542 Carter Oct. 15, 1946 2,419,552 Himmel Apr. 29, 1947 FOREIGN PATENTS 916,270 France December 1946 

